1. Field of Invention
The present invention relates to the field of preparation of high molecular foam materials, and in particular to a method for producing a microcellular foam polypropylene thick board.
2. Related Art
A microcellular foam polymeric material refers to a porous foam polymeric material with cell size of less than 100 μm and cell density of greater than 1.0×106/cm3. Due to its light weight, high strength, and material saving properties etc., the microcellular foam materials have widespread prospects in application. Among a multitude of the microcellular foam polymeric materials, the microcellular foam polypropylene (PP) materials have favorable mechanical properties, higher heat-deformation temperature, chemical resistance, surface protection performance and easy recycle and reuse. For the microcellular foam polypropylene materials obtained by foaming with supercritical carbon dioxide or supercritical nitrogen gas, it is clean, and pollution-free. In particular, the microcellular foam polypropylene board with high thickness in articles, for example, a thickness of more than 10 mm, which is the desirable core material for a sandwiching composite, may be used as a thermal barrier in a thermally insulated car and a refrigerator car, a ceiling board and floor in an automobile, a passenger car and a railway transportation vehicle, etc., and may be used for thermal insulation of ships and buildings etc.
It is difficult for the common continuous extrusion foaming method to make the microcellular foam polypropylene thick board. With limitation by the dispersion level of the raw material resin and gas in the molten mass, the depressurization rate at the die head, the removal of heat at the core of foam, and the like, the continuous extrusion foaming is only suitable for production of the microcellular foam board with relatively lower thickness of generally less than 5 mm.
The solid foaming method is commonly used for preparation of the microcellular foam polymeric material. The solid foaming process is one in which at the condition of the polymer still being in the form of solid with the foaming temperature being lower than the flowing temperature, the high pressure gas is dissolved and diffused into the polymeric matrix prior to induction of cell nucleation and growth by flash blow-down. In general, during solid foaming, the foaming temperature is higher than the glass transition temperature for the amorphous polymers, and is lower than the melting point for the semi-crystal polymers.
The solid foaming process is characterized in that: (1) a low requirement for foamability of the raw material resin, which is different from the foaming processes such as extrusion, injection molding and the like. For providing the foam material with closed cell structure, the material is usually required to have higher strength of molten mass for fusion foaming, to avoid cell fracture phenomena at the time of cell growth. For solid foaming process, as a result of the polymer itself being in the form of solid, the polymer has its own sufficient strength to allow the tensile effect on the polymer during cell growth and maintain the closed cell structure of cells. Therefore, the general polymers are available for foaming, without the foaming raw materials with high molten mass strength obtained from special modification. (2) during solid foaming, the gas passes by self-diffusion into the polymeric matrix for dispersion at the molecular level, in favor of uniform nucleation of cells, thus facilitating formation of the microcellular foaming material with small cell size and high cell density. (3) Temperature uniformity within the polymer at pressurized atmosphere and readily available flash blow-down tend to provide the microcellular foaming material with small cell size, high cell density and uniform cell size.
During solid foaming, the high pressure gas permeates by self-diffusion in the polymeric matrix, and the saturation time for diffusion equilibrium depends on the foaming temperature, the gas pressure, the type of polymer resins and its thickness. For most of polymers, the gas usually diffuses therein at a slower rate, and the saturation time required for dissolution equilibrium is long. Especially with increase of the articles in thickness, the saturation time would extend significantly. Therefore, the solid foaming method is often used for preparation of the board with lower thickness, but it is time-consuming and inefficient for preparation of the microcellular foam board with higher thickness.